U.S. patent number 10,615,606 [Application Number 16/108,852] was granted by the patent office on 2020-04-07 for circuit for voltage limitation in a photovoltaic field, photovoltaic field, and method for voltage limitation.
This patent grant is currently assigned to SMA Solar Technology AG. The grantee listed for this patent is SMA Solar Technology AG. Invention is credited to Andreas Falk, Karl Nesemann.
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United States Patent |
10,615,606 |
Nesemann , et al. |
April 7, 2020 |
Circuit for voltage limitation in a photovoltaic field,
photovoltaic field, and method for voltage limitation
Abstract
A circuit for limiting the voltage of a photovoltaic string
divided into a first section and a second section of a series
circuit of solar modules, includes a first terminal for connection
to a first end of the first section, a second terminal for
connection to a first end of the second section and a third
terminal for connection to a second end of the second section. The
circuit also includes a bypass switch, which is connected at one
end to the first terminal and at the other end to the third
terminal and a disconnect switch, which is connected at one end to
the first terminal and at the other end to the second terminal. The
circuit has a controller for actuating the bypass switch and the
disconnect switch, wherein the controller is configured to
determine a first threshold value U.sub.UL and a second, lower
threshold value U.sub.LL depending on a switch voltage U.sub.OC
dropped across the disconnect switch in an open state and taking
into account the number of solar modules in the first section and
in the second section.
Inventors: |
Nesemann; Karl (Kaufungen,
DE), Falk; Andreas (Kassel, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
SMA Solar Technology AG |
Niestetal |
N/A |
DE |
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Assignee: |
SMA Solar Technology AG
(Niestetal, DE)
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Family
ID: |
60935817 |
Appl.
No.: |
16/108,852 |
Filed: |
August 22, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190013420 A1 |
Jan 10, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/EP2017/083146 |
Dec 15, 2017 |
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Foreign Application Priority Data
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Dec 21, 2016 [DE] |
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10 2016 125 219 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02S
40/36 (20141201); H01L 31/02021 (20130101); H02J
3/38 (20130101); H02J 3/383 (20130101); H02S
50/00 (20130101) |
Current International
Class: |
H02J
3/38 (20060101); H02S 50/00 (20140101); H01L
31/02 (20060101); H02S 40/36 (20140101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102009044695 |
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Jun 2011 |
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DE |
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2362520 |
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Aug 2011 |
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EP |
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S6017516 |
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Jan 1985 |
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JP |
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2015092441 |
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Jun 2015 |
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WO |
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Other References
International Search Report dated Apr. 16, 2018 in connection with
International Application PCT/EP2017/083146. cited by
applicant.
|
Primary Examiner: Amaya; Carlos
Attorney, Agent or Firm: Eschweiler & Potashnik, LLC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of International Patent
Application number PCT/EP2017/083146, filed on Dec. 15, 2017, which
claims priority to German Patent Application number 10 2016 125
219.0, filed on Dec. 21, 2016, and is hereby incorporated by
reference in its entirety.
Claims
The invention claimed is:
1. A circuit for limiting the voltage of a photovoltaic string
divided into a first section and a second section of a series
circuit of solar modules, comprising: a first terminal configured
to connect to a first end of the first section, a second terminal
configured to connect to a first end of the second section, and a
third terminal configured to connect to a second end of the second
section, a bypass switch connected at one end to the first
terminal, and at the other end to the third terminal, a disconnect
switch connected at one end to the first terminal and at the other
end to the second terminal, and a controller configured to actuate
the bypass switch and the disconnect switch, wherein the controller
is configured to determine a first threshold value U.sub.UL and a
second, lower threshold value U.sub.LL depending on a switch
voltage U.sub.OC dropped across the disconnect switch in an open
state and taking into account a number of solar modules in the
first section and in the second section, and further configured to
compare a module voltage U.sub.S dropped between the first terminal
and a voltage input of the circuit with the threshold values
U.sub.UL, U.sub.LL, in order to open the disconnect switch and to
close the bypass switch when the first threshold value U.sub.UL is
exceeded by the module voltage U.sub.S, and to open the bypass
switch and to close the disconnect switch when the second threshold
value U.sub.LL is undershot by the voltage U.sub.S.
2. The circuit as claimed in claim 1, wherein the circuit is
configured to supply energy from the module voltage U.sub.S.
3. The circuit as claimed in claim 1, wherein upon a calibration
condition being met, the controller, with the disconnect switch
closed, calibrates the threshold values U.sub.UL, U.sub.LL by
opening the disconnect switch, determining the switch voltage
U.sub.OC dropped across the opened disconnect switch, and
determining updated threshold values U.sub.UL, U.sub.LL under such
conditions.
4. The circuit as claimed in claim 3, wherein the condition
comprises an expiration of a prescribed period since the last
closure of the disconnect switch.
5. The circuit as claimed in claim 3, wherein the condition
comprises an exceeding of a prescribed change threshold value by a
difference of the module voltage U.sub.S between the present value
and the value determined after the last closure of the disconnect
switch.
6. A photovoltaic field comprising a plurality of strings connected
in parallel between a first and a second DC voltage bus, the
strings comprising a first section and a second section of series
circuits of solar modules, wherein the sections of each respective
string are connected to one another by means of a circuit, the
circuit comprising: a first terminal configured to connect to a
first end of the first section, a second terminal configured to
connect to a first end of the second section, and a third terminal
configured to connect to a second end of the second section, a
bypass switch connected at one end to the first terminal, and at
the other end to the third terminal, a disconnect switch connected
at one end to the first terminal and at the other end to the second
terminal, and a controller configured to actuate the bypass switch
and the disconnect switch, wherein the controller is configured to
determine a first threshold value U.sub.UL and a second, lower
threshold value U.sub.LL depending on a switch voltage U.sub.OC
dropped across the disconnect switch in an open state and taking
into account a number of solar modules in the first section and in
the second section, and further configured to compare a module
voltage U.sub.S dropped between the first terminal and a voltage
input of the circuit with the threshold values U.sub.UL, U.sub.LL,
in order to open the disconnect switch and to close the bypass
switch when the first threshold value U.sub.UL is exceeded by the
module voltage U.sub.S, and to open the bypass switch and to close
the disconnect switch when the second threshold value U.sub.LL is
undershot by the voltage U.sub.S, wherein in each case one end of
the first section is connected to the first DC voltage bus and the
other end of the first section is connected to the first terminal
of the circuit, wherein one end of the second section is connected
to the second terminal and the other end of the second section is
connected to the second DC voltage bus and the third terminal of
the circuit, and wherein the voltage input is connected to a
terminal of a solar module of the first section being opposite to
the first terminal of the circuit.
7. The photovoltaic field as claimed in claim 6, wherein the
voltage input is connected to the terminal of a solar module of the
first section that is connected directly to the first terminal of
the circuit using the other terminal.
8. The photovoltaic field as claimed in claim 6, wherein a number
of solar modules of the first section is selected such that, in the
case of a lowest specified ambient temperature and a highest
specified insolation, an open-circuit voltage of the solar modules
still remains below a prescribed maximum value, and a number of
solar modules of the second section is selected such that, in the
case of the lowest specified ambient temperature, an MPP voltage of
the series circuit of the first and second section still remains
below the prescribed maximum value.
9. The photovoltaic field as claimed in claim 8, wherein the
prescribed maximum value is greater than 1000 volts.
10. The photovoltaic field as claimed in claim 6, wherein the other
end of the second section is connected to the second DC voltage bus
by means of a third section of solar modules.
11. The photovoltaic field as claimed in claim 6, wherein in each
circuit of the parallel strings an offset that is different from
one another is stored therein, which offset causes the respective
controllers of the circuits to determine different threshold values
U.sub.LL, U.sub.LL in the case of an identical determined switch
voltage U.sub.OC.
12. A method for limiting the voltage of a photovoltaic string,
wherein the string comprises a first section of solar modules and a
second section of solar modules, wherein a bypass switch is
arranged in parallel with the second section and a disconnect
switch is arranged in series with the solar modules of the second
section, comprising: determining a module voltage U.sub.S within
the first section and a switch voltage U.sub.OC dropped across the
disconnect switch while the disconnect switch is open and the
bypass switch is closed, determining a first threshold value
U.sub.UL and a second, lower threshold value U.sub.LL depending on
the switch voltage U.sub.OC and taking into account a number of
solar modules in the first section and a number of solar modules in
the second section, and closing the bypass switch and opening the
disconnect switch to establish a short string state when the
presently determined module voltage U.sub.S exceeds the first
threshold value U.sub.UL, and opening the bypass switch and closing
the disconnect switch to establish a long string state when the
presently determined module voltage U.sub.S undershoots the second
threshold value U.sub.LL.
13. The method as claimed in claim 12, wherein determining the
module voltage U.sub.S and the switch voltage U.sub.OC are
repeated, and the first threshold value U.sub.UL and the second,
lower threshold value U.sub.LL are determined anew with every
repetition.
14. The method as claimed in claim 12, wherein a condition for
recalibration is checked during the long string state, and wherein,
upon a meeting of the condition, the switch position of the bypass
switch and the disconnect switch change briefly to the short string
state in order to determine the switch voltage U.sub.OC dropped
across the disconnect switch and to determine updated threshold
values U.sub.UL, U.sub.LL therefrom.
15. The method as claimed in claim 14, wherein the condition
comprises the expiration of a waiting time, in which the string was
in a long string state, wherein the waiting time is greater than
100 seconds.
16. The method as claimed in claim 14, wherein the condition
comprises an exceeding of a prescribed change threshold value by
the difference of the module voltage U.sub.S between the current
value and the value directly after the last change to the long
string state.
17. The method as claimed in claim 12, wherein, after the change to
the short string state, the string remains in this state for a
minimum time period, wherein the minimum time period is greater
than 2 ms.
18. The method as claimed in claim 12, wherein the second threshold
value U.sub.LL is determined from the first threshold value
U.sub.UL using a value table.
19. The method as claimed in claim 12, wherein permissible value
ranges for the first threshold value U.sub.UL and the second
threshold value U.sub.LL are prescribed, wherein, in the case of
deviation from the respective permissible value range, the
threshold value is set to the respective limit value of the value
range.
Description
FIELD
The disclosure relates to a circuit for limiting the voltage of a
photovoltaic string, to a photovoltaic field having a plurality of
parallel strings, which have a circuit of this kind, and to a
method for limiting the voltage of a photovoltaic string.
BACKGROUND
In order to be able to design large photovoltaic energy generation
installations in an efficient manner, it is desirable to connect as
many solar modules as possible in series so that the series
circuit, what is known as a string, has a very high voltage. In
order not to exceed the permissible input voltage range of a
connected inverter or the permissible module voltage with respect
to ground potential, the length of such a string has to be limited
so that the open-circuit voltage thereof does not exceed the
maximum permissible value, even in the case of extreme conditions,
in particular at low temperatures and high insolation.
Alternatively, it is known from the prior art to provide circuits
that disconnect or short-circuit some of the modules when a maximum
string voltage is exceeded in order to achieve a reduction in the
voltage. After the string voltage has dropped again to a sufficient
extent, the short-circuit or the disconnection can be discontinued
again. One example of such a circuit is disclosed in document US
2013/0049710 A1, in which, for the purpose of voltage limitation,
two or more solar modules of a string are bypassed by means of a
relay when an open-circuit state of the string is identified.
Document DE 10 2010 009 120 A1, for example, further discloses a
photovoltaic field of parallel strings, wherein some of the
parallel strings can be short-circuited jointly by means of a
short-circuiting switch, which is activated when a prescribed
voltage is exceeded by the string. Since the short-circuiting of a
portion of a string constitutes a loss of performance for the
energy generation installations, it is desirable to be able to
shorten the strings individually in order to have to shorten only
the number of strings required in the present situation. In this
case, however, it must be ensured that the shortened strings are
not brought into a reverse current situation due to the long
strings, in which reverse current situation, in the short strings,
a current magnitude that significantly exceeds the rated current of
the string flows in the reverse direction and can damage the
strings.
SUMMARY
The present disclosure is directed to a circuit and a method,
respectively, for limiting the voltage of a string, which circuit
and method are capable of shortening the string when a threshold
value is exceeded by the string voltage, wherein the threshold
value is determined by the circuit so that damaging reverse current
situations are prevented. The disclosure is further directed to a
photovoltaic field having a plurality of parallel strings each
having such a circuit, wherein the circuits of the individual
strings operate independently of one another.
A circuit according to the disclosure for limiting the voltage of a
photovoltaic string, which is divided into a first section and a
second section of a series circuit of solar modules, comprises a
first terminal for connection to a first end of the first section,
a second terminal for connection to a first end of the second
section and a third terminal for connection to a second end of the
second section. The circuit also comprises a bypass switch, which
is connected at one end to the first terminal and at the other end
to the third terminal, and a disconnect switch, which is connected
at one end to the first terminal and at the other end to the second
terminal. A controller of the circuit serves to actuate the bypass
switch and the disconnect switch, wherein the controller is
configured to determine a first threshold value U.sub.UL and a
second, lower threshold value U.sub.LL depending on a switch
voltage U.sub.OC dropped across the disconnect switch in an open
state and taking into account the number of solar modules in the
first section and in the second section The controller is also
configured to compare a module voltage U.sub.S dropped between the
first terminal and a voltage input of the circuit with the
threshold values U.sub.UL, U.sub.LL, in order to open the
disconnect switch and to close the bypass switch when the first
threshold value U.sub.UL is exceeded by the module voltage U.sub.S,
and to open the bypass switch and to close the disconnect switch
when the second threshold value U.sub.LL is undershot by the
voltage U.sub.S.
The circuit according to the disclosure can electrically connect
the sections to one another or isolate them from one another
depending on the module voltage U.sub.S so that, in the case of a
sufficiently low module voltage and thus a string voltage that can
be determined therefrom, the first section and the second section
can be connected in series--referred to in the following text as
the long state of the string--whereas, in the case of such a high
module voltage that a series circuit of the sections would lead to
an undesirably high string voltage, the second section is
disconnected and bypassed and thus does not contribute to the
string voltage--referred to in the following text as the short
state of the string. In this way, the circuit ensures limitation of
the string voltage without reducing the maximum power that can be
achieved by the string. In this case, the circuit can also be
integrated into existing strings retrospectively, since the string
has to be divided only into a first and a second section for this
and the terminals of the circuit have to be connected to
corresponding terminal points of the string.
The circuit is advantageously configured to supply energy from the
module voltage U.sub.S and is thus not assigned to its own energy
source. To this end, the circuit draws the energy required for
operation from the voltage applied between the first terminal and
the voltage input. If necessary, an energy store, for example a
capacitor, supercapacitor or a chargeable battery, is provided in
order to supply power to the circuit even at times at which the
applied module voltage U.sub.S is insufficient.
The disconnect switch and the bypass switch are, in one embodiment,
provided as semiconductor switches, in particular as field-effect
transistors or bipolar transistors, more specifically as IGBTs or
MOSFETs, and can be of the same type or of different types. An
intrinsic or a separate antiparallel freewheeling diode can be
provided with the switches.
On account of the provided connection of the terminals of the
circuit, the voltage dropped across the opened disconnect switch
corresponds to the open-circuit voltage of the second section if
the bypass switch is closed or, on account of a freewheeling diode
associated with the bypass switch, no noteworthy voltage is dropped
across the bypass switch. The measurement of the voltage dropped
across the opened disconnect switch therefore permits determination
of the open-circuit voltage of the solar modules and of the string.
From this knowledge, it is possible to derive directly, whether
and, if applicable, to what extent the string voltage exceeds an
open-circuit voltage of the first section so that a reverse current
can arise when the second section is isolated from or bypassed by
the first section. The second threshold value U.sub.LL can be
selected accordingly so that reverse currents are prevented or at
least limited to a safe extent.
At this point, it should be advised that, for the determination of
suitable threshold values U.sub.UL, U.sub.LL, the knowledge of the
number of solar modules in the first section and the number of
solar modules in the second section as well as the number of solar
modules by means of which the module voltage U.sub.S is determined
have to be stored in the controller. The values can be stored as
absolute or as relative values. With knowledge of these values, it
is possible to convert the module voltage U.sub.S directly to a
voltage dropped across the first section or to a string
voltage.
When the disconnect switch is closed and the open-circuit voltage
of the second section cannot be determined as a voltage dropped
across the disconnect switch, it is advantageous to provide a
condition in the controller, the meeting of which condition is
checked continuously or regularly. The condition describes a
necessity of checking the open-circuit voltage of the second
section to determine whether the open-circuit voltage has changed
and of updating the threshold values U.sub.UL, U.sub.LL. When the
condition is met when the disconnect switch is closed, the
controller updates the threshold values U.sub.UL, U.sub.LL by
virtue of the disconnect switch being briefly opened and the switch
voltage U.sub.OC dropped across the opened disconnect switch being
determined. The updated threshold values are determined from the
determined switch voltage. The condition can comprise an expiry of
a prescribed period since the last closure of the disconnect
switch. As an alternative or in addition, the condition can also
comprise an exceeding of a prescribed change threshold value by the
difference of the module voltage U.sub.S between the present value
and the value determined directly after the last closure of the
disconnect switch. A condition that depends on a change in
temperature of the solar modules since the last closure of the
disconnect switch is also conceivable.
In a further aspect, a photovoltaic field comprises a plurality of
strings connected in parallel between a first and a second DC
voltage bus, which strings each have a first section and a second
section of series circuits of solar modules. The first and the
second section are connected to one another by means of a circuit
according to the disclosure described above, wherein in each case
one end of the first section is connected to the first DC voltage
bus and the other end of the first section is connected to the
first terminal of the circuit. At the same time, one end of the
second section is connected to the second terminal of the circuit
and the other end of the second section is connected to the second
DC voltage bus. The third terminal of the circuit is likewise
connected to the other end of the second section, while the voltage
input is connected to a terminal of a solar module of the first
section opposite to the first terminal of the circuit, so that the
voltage of a portion of the first section or of the entire first
section is dropped between the first terminal of the circuit and
the voltage input and can be determined as the module voltage
U.sub.S. In order to keep the module voltage low, the voltage input
can, in one embodiment, be connected to the terminal of that solar
module of the first section that is connected directly to the first
terminal of the circuit using the other terminal. In this way, the
module voltage U.sub.S corresponds to the voltage of a single solar
module.
In order to adjust the photovoltaic field to the input voltage
range of a connected inverter in optimum fashion, the number of
solar modules of the first section can be selected in such a way
that, in the case of a lowest specified ambient temperature and a
highest specified insolation, the open-circuit voltage of the solar
modules still remains below a prescribed maximum value, and the
number of solar modules of the second section can be selected in
such a way that, under the same specified extreme conditions, an
MPP voltage of the series circuit of the first and second section
still remains below the specified maximum value. In this case, the
maximum value corresponds to the maximum input voltage of the
inverter or to the specified maximum voltage of a component of the
photovoltaic field, for example to the maximum insulation voltage
of the solar modules. This can result in a situation in which the
inverter can remain connected to the photovoltaic field even in the
case of the specified extreme conditions in the open-circuit state.
The maximum value is preferably greater than 1000 volts, in
particular 1500 volts, and thus corresponds to common
specifications of commercially available inverters for rural
installations.
The other end of the second section can be connected directly to
the second DC voltage bus. However, a third section of solar
modules can also be arranged between the other end and the DC
voltage bus so that the second section that can be bypassed is
arranged between two sections that cannot be bypassed. Such a third
section of solar modules is taken into account during the
determination of the number of solar modules of the first section
so that subsequently reference can no longer be made explicitly to
the number of solar modules in the third section because this
number is concomitantly counted in the first section.
In a photovoltaic field comprising a plurality of parallel strings
of which the voltage is limited by the circuit according to the
disclosure, it may be advantageous to ensure that the strings do
not change between the long and the short state simultaneously, if
possible. This contributes to voltage-stable operation of the
photovoltaic field. This can be achieved by storing in each case an
offset that is different from the other string in the circuits of
the parallel strings, which offset causes the controllers of the
circuits to determine different threshold values U.sub.UL, U.sub.LL
in the case of an identical determined switch voltage U.sub.OC.
Threshold values that are different from one another in the various
circuits of the strings are thus generally also determined when the
temperature conditions and insolation conditions between the
strings are identical. Different points in time of the change
between the long and short state of the string are achieved
accordingly.
A method according to the disclosure for limiting the voltage of a
photovoltaic string, which has comprising a first section and a
second section of solar modules, wherein a bypass switch is
arranged in parallel with the second section and a disconnect
switch is arranged in series with the solar modules of the second
section, comprises repeatedly determining a module voltage U.sub.S
within the first section and a switch voltage U.sub.OC dropped
across the disconnect switch while the disconnect switch is open
and the bypass switch is closed. A first threshold value U.sub.UL
and a second, lower threshold value U.sub.LL are determined
depending on the switch voltage U.sub.OC and taking into account a
number of solar modules in the first section and in the second
section. The bypass switch is closed and the disconnect switch is
opened to produce a short state of the string when the currently
determined module voltage U.sub.S exceeds the first threshold value
U.sub.UL. The bypass switch is opened and the disconnect switch is
closed to produce a long state of the string when the currently
determined module voltage U.sub.S undershoots the second threshold
value U.sub.LL. In this way, the string can autonomously change in
good time to the short state in order to prevent an excessive
string voltage and change back to the long state when the string
voltage has dropped to a sufficient extent. In particular, the
method works without communication, for example to a superordinate
control device.
In this case, the threshold values are not fixedly prescribed but
are adjusted flexibly to the prevailing operating conditions, for
example to the prevailing temperature and insolation. In this case,
the adjustment does not even require appropriate sensors since the
determined voltages already contain sufficient information about
the prevailing conditions. Nevertheless, the inclusion of further
sensor data, for example from a temperature or an insolation sensor
or from a current sensor for determining the string current, should
not be excluded since this inclusion can further improve the
method. The fact that voltage values determined on other strings,
in particular voltage values obtained in the case of a parallel
execution of the method according to the disclosure, are taken into
account in the determination of the threshold values should also
not be excluded, even though this restricts the advantage of the
autonomy of the change of state.
During the change from the long state to the short state, the
disconnect switch is, in one embodiment, first opened before the
bypass switch is closed. In an analogous manner, during the change
from the short state to the long state, the bypass switch is first
opened and then the disconnect switch is closed. It should
furthermore be ensured that the disconnect switch was opened for a
sufficient time before the voltage dropped across it is determined
in order that the voltage determined in this way also constitutes
the open-circuit voltage of the second section. In one embodiment
of the disclosure, a waiting time of 1.mu.s is provided between the
switching of the disconnect switch and the switching of the bypass
switch. In a further embodiment, after the disconnect switch has
been opened, a waiting time of between 200 .mu.s and 10 ms can be
provided before the voltage dropped across the disconnect switch is
determined.
When the string remains in the long state for a long time, it may
be expedient to recalibrate the threshold values in order to meet
the time of the change to the short state in optimum fashion. To
this end, a condition for recalibration is checked while the string
is in the long state. When the condition is met, the switch
position briefly changes to the short state in order to determine
the switch voltage U.sub.OC dropped across the disconnect switch
and to determine updated threshold values U.sub.UL, U.sub.LL
therefrom. An updated module voltage U.sub.S is, in one embodiment,
determined at the same time and compared with the updated threshold
value U.sub.UL. If the updated module voltage U.sub.S is below the
updated threshold value U.sub.UL, there is a change back to the
long state; otherwise, the string remains in the short state.
The condition for recalibration can comprise the expiry of a
waiting time in which the string was in a long state. In one
embodiment, a waiting time between 100 seconds and 30 minutes is
used. The waiting time should be selected so that the prevailing
operating conditions during this time do not change
significantly.
In one variant of the method, the condition for the recalibration
comprises an exceeding of a prescribed change threshold value by
the difference of the module voltage U.sub.S between the actual
value and the value directly after the last change to the long
state. Through the exceeding of the change threshold value, it is
thus possible to detect a significant change in the prevailing
operating conditions. Of course, it is conceivable to link a
plurality of partial conditions, for example the two condition
examples described above, through logic combination with the
condition for the recalibration.
In one embodiment of the method according to the disclosure, the
first threshold value U.sub.UL is determined first, for example by
virtue of a functional correlation between the module voltage
U.sub.S and/or the voltage U.sub.OC dropped across the disconnect
switch as functional parameters and the first threshold value
U.sub.UL=f(U.sub.S, U.sub.OC) being taken as a basis. It is
likewise conceivable to determine the first threshold value
U.sub.UL by means of a value table, where appropriate with
interpolation between the values of the value table. The second
threshold value can be determined in principle by means of a second
functional correlation or a second value table in an analogous
manner. However, it is particularly advantageous to determine the
second threshold value U.sub.LL only from the first threshold value
U.sub.UL that has already been determined. This determination can
also take a third functional correlation U.sub.LL=g(U.sub.UL) or a
third value table as a basis.
In order to prevent undesirably large or small threshold values, in
each case permissible value ranges for the first threshold value
U.sub.UL and the second threshold value U.sub.LL can be prescribed,
wherein, in the case of departure from the respective permissible
value range, the threshold value is set to the respective limit
value of the value range.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following text, the disclosure is illustrated using figures,
in which
FIG. 1 shows a circuit according to the disclosure for limiting the
voltage of a string,
FIG. 2 shows a photovoltaic field according to the disclosure
comprising parallel strings comprising a voltage-limiting
circuit,
FIG. 2a shows a further photovoltaic field according to the
disclosure comprising parallel strings comprising a
voltage-limiting circuit,
FIG. 3 shows a flowchart of a method according to the disclosure
for limiting the voltage of strings, and
FIG. 4 shows a current-voltage characteristic curve profile of a
photovoltaic field according to the disclosure.
DETAILED DESCRIPTION
FIG. 1 shows the design of a circuit 1 for connection to sections
of a photovoltaic string. A first terminal 13 of the circuit 1, to
which first terminal one end of a first section of a photovoltaic
string can be connected, is connected to a second terminal 14 of
the circuit 1 by means of a disconnect switch 11, to which second
terminal one end of a second section of the string can be
connected. In this way, an electrical connection between the first
section and the second section can be controlled by means of the
disconnect switch 11.
At the same time, the first terminal 13 is connected to a third
terminal 15 of the circuit 1 by means of a bypass switch 12, which
third terminal can be connected to the other end of the second
section of the string. In this way, the second section can be
bypassed in a controlled manner by means of the bypass switch 12 so
that a current flowing through the first section can be conducted
around the second section. In this way, it is possible to achieve a
situation in which the string can be actuated in such a way that,
in one switch position of the disconnect switch 11 and of the
bypass switch 12, the string has a length that corresponds to the
length of the first section and, in a further switch position, has
a length that corresponds to the sum of the lengths of the first
and the second section.
A controller 10 can open and close the disconnect switch 11 and the
bypass switch 12 for changing the string length by means of
corresponding control lines. To generate the switch-on and
switch-off signals, the controller 10 has a first voltage input 17
and a second voltage input 18, by means of which a switch voltage
U.sub.OC dropped across the disconnect switch 11 can be determined.
In the case shown, the first voltage input 17 is connected to the
first terminal 13 and the second voltage terminal 18 is connected
to the second terminal 14. As a conceivable alternative, the second
voltage input 18 can also be connected to the third terminal 15 and
can thus determine the voltage dropped jointly across the
disconnect switch 11 and the bypass switch 12.
When the string sections are connected as described above, the
open-circuit voltage of the second section is dropped across the
disconnect switch 11 when it is open. Furthermore, the controller
10 is connected to a third voltage input 16, which is connected to
a connecting point between solar modules of the first section so
that a module voltage U.sub.S of the first section of the string is
dropped between the third voltage input 16 and the first voltage
input 17 and can be determined. The third voltage input 16 is, in
one embodiment, connected to the end opposite to the first terminal
13, of the solar module of the first section of the string, which
solar module is connected directly to the circuit 1, so that the
module voltage U.sub.S constitutes the present voltage of the solar
module. As an alternative, it is also conceivable, of course, to
connect the third voltage input 16 to another point of the series
circuit of the solar modules of the first section 21 in order to
thus determine the voltage across a plurality of solar modules or
even across the entire first section 21 in this way.
FIG. 2 shows a photovoltaic field 2 comprising a plurality of
strings 20 connected in parallel, which strings each have a circuit
1 according to the disclosure. A first section 21 of a series
circuit of solar modules of the string 20 is connected with a first
end to the first terminal 13 of the circuit 1 and with another end
to a positive DC bus DC+. The third voltage input 16 of the circuit
1 is connected to an end of a solar module 24, which end is remote
from the first terminal 13, in order that the circuit 1 can
determine the module voltage at the solar module 24. One end of a
second section 22 of a series circuit of solar modules of the
string 20 is connected at the second terminal 14 of the circuit 1,
while the other end is connected to a negative DC bus DC-. The
third terminal 15 of the circuit 1 is also connected to the
negative DC bus DC-.
The photovoltaic field 2 of FIG. 2a differs from that of FIG. 2 in
that a third section 23 of solar modules is provided, which third
section cannot be bypassed and which is arranged between the second
section 22 and the negative DC bus DC-. In this arrangement, too,
the circuit 1 can be used for voltage limitation.
A flowchart of a method according to the disclosure for reducing
the voltage of a photovoltaic string is shown in FIG. 3. At the
beginning of the method, the string is in a long state in
accordance with act 30, that is to say that the first section and
the second section are connected in series with one another. If
necessary, the string is transferred to a long state through
opening of the bypass switch 12 and closure of the disconnect
switch 11. In this state, at 31, a check is carried out to
determine whether a condition for recalibration of the threshold
values is met. Reference is made once again to this condition and
the acts undertaken when the condition is met in a later paragraph
of the description. Subsequently, at 32, the module voltage U.sub.S
is determined, wherein, where necessary, high-frequency components
are removed by forming the mean value of a plurality of
measurements or by low-pass filtering from the measurement result.
Even when the module voltage U.sub.S is determined only at a
portion of the first section or even only at a single solar module
of the first section, it is assumed, for the sake of simplicity of
the illustration, that the module voltage U.sub.S subsequently
relates to the total voltage of the first section. If necessary,
the determined value is provided with a suitable scaling factor,
which takes into account the number of solar modules in the first
and second section. The determined value of the module voltage
U.sub.S is compared at 33 with the present first threshold value
U.sub.UL. If the module voltage U.sub.S is lower than the first
threshold value U.sub.UL (+), there is a return to act 31.
However, if the module voltage U.sub.S exceeds the first threshold
value U.sub.UL (-), a short state of the string is established at
34 by virtue of the bypass switch being closed and the disconnect
switch being opened. Subsequently, at 35, the switch voltage
U.sub.OC dropped across the disconnect switch is determined, which
voltage is a measure of the open-circuit voltage of the solar
modules and substantially corresponds to the open-circuit voltage
of the second section. At 36, a current first threshold value
U.sub.UL and a second, lower threshold value U.sub.LL are then
determined from the switch voltage U.sub.OC determined in this way.
During this determination, the number of solar modules of the first
section and of the second section is concomitantly taken into
account.
In one embodiment, the following formulae are used for ascertaining
the threshold values:
.function..function. ##EQU00001##
In this case, n is the number of solar modules in the first section
and m is the number of solar modules in the second section. The
functions f.sub.1 and f.sub.2 can be defined, for example, by means
of a look-up table. However, it is also conceivable to provide one
or both functions f.sub.1, f.sub.2 as linear functions of the
open-circuit voltage U.sub.OC. The function f.sub.1 is intended to
be dimensioned independently thereof in such a way that the
threshold value U.sub.UL is as high as possible but still not so
low that the first section of the string is not exposed to a
dangerously high reverse current due to the applied voltage in the
case of transferral to the short state. The threshold value
U.sub.UL is advantageously selected so that no reverse current
occurs or so that it is ensured that the reverse current remains
lower than the rated current of the solar module. It is furthermore
recommended to provide upper and/or lower limit values for the
threshold values U.sub.UL, U.sub.LL, which limit values define a
value range that the threshold values are not intended to
leave.
If it is desired for the circuits for voltage limitation in
parallel strings not to switch between the strings at the same time
even under identical temperature conditions and insolation
conditions, it is advantageous to provide an offset value, which is
determined differently between the parallel strings, for example as
a random number. The offset value can be added to the threshold
values determined according to the above formula.
Returning to the further description of the method according to the
disclosure, at 37, after the updated threshold values U.sub.UL,
U.sub.LL have been determined, the module voltage U.sub.S is
determined again, wherein the determined module voltage now relates
only to the voltage of the first section of the string. The module
voltage U.sub.S determined in this way is compared with the second
threshold value U.sub.LL. If the module voltage U.sub.S exceeds the
second threshold value U.sub.LL (-), there is a return to act 35
and the string remains in the short state. Otherwise (+), the
method branches off to act 30 and the string is transferred back to
a long state.
Since the open-circuit voltage cannot be determined as the switch
voltage U.sub.OC dropped across the disconnect switch in a long
state of the string, there is a check at 31 described above as to
whether a condition for recalibration of the threshold values is
met. The condition can comprise the expiration of a clock since the
last determination of updated threshold values. However, the
condition can also comprise a change criterion for the module
voltage. For example, it is possible to check whether the present
module voltage has changed compared to the determined module
voltage during the last change to the long state by a prescribed
percentage or a prescribed absolute value. Of course, a logic
combination of a plurality of conditions, in particular an OR
combination, is also conceivable.
If the condition at 31 is met (+) the string is transferred to the
short state at 40 so that, at 41, the switch voltage U.sub.OC
dropped across the disconnect switch is determined and, at 42,
current threshold values U.sub.UL, U.sub.LL are determined
therefrom. After this, the string is immediately transferred back
to the long state at 43 and the method continues at 32. Act 40 can
be carried out analogously to act 34, act 41 can be carried out
analogously to act 35, act 42 can be carried out analogously to act
36, and act 43 can be carried out analogously to act 30.
FIG. 4 shows a characteristic curve of a photovoltaic field
comprising five strings connected in parallel, where each have a
circuit according to the disclosure for voltage limitation, and
illustrates simultaneously the functioning of the method according
to the disclosure. The voltage of the strings connected in parallel
is shown on the horizontal axis and the sum of the string currents
is shown on the vertical axis. The characteristic curve relates to
a string configuration of 32 solar modules in the first section and
4 solar modules in the second section, each with an open-circuit
voltage of 46.1 V and a short-circuit current of 8.33 A. An
insolation of 1000 W/m.sup.2 and a cell temperature of 10.degree.
C. have been taken as a basis. This produces an MPP power of the
string of 9927 W at an MPP voltage of 1296 V.
At a low string voltage as the output voltage, in this case lower
than 1300 V, all of the strings of the photovoltaic field are in a
long state. As the voltage increases, the characteristic curve
shown follows an upper profile 45 up to a voltage U.sub.UL,1 of
1430 V. At this voltage, the first string is transferred to a short
state, as a result of which a negative current step is produced. In
the case of a further increasing voltage, a further string is
transferred in each case to the short state every 10 V (see current
steps at the voltage values U.sub.UL,2 . . . 5) until, at a voltage
above 1470 V, all of the strings are in the short state. This
ensures that the photovoltaic field does not exceed a voltage of
1475 V even in an open circuit so that an inverter with a maximum
permissible input voltage of 1500 V can remain connected to the
photovoltaic field safely even in this case although the
open-circuit voltage of all of the solar modules of the first and
the second section is 1660 V (=36*46.1 V).
If the string voltage decreases starting from the open-circuit
voltage at 1475 V, the characteristic curve follows a lower profile
46, in which all of the strings up to a voltage U.sub.LL,5 of 1340
V are in a short state. At this voltage and every 10 V in the case
of a further decreasing voltage (see further voltage step levels
U.sub.LL,1 . . . 4), in each case one of the strings is transferred
back to a long state so that five positive voltage step levels are
produced, until the characteristic curve at a voltage of 1300 V
reaches the upper profile 45 again.
The position of the voltage step levels does not have to be
designed to be equidistant here, not even fixedly prescribed. In
one embodiment, each string ascertains within its circuit for
voltage limitation its own threshold values independently of the
other strings, for example on the basis of the method described
above. However, it is advantageous if the voltage range 48 in which
the strings are successively transferred to the short state is
separate from the voltage range 47 in which the strings are
successively transferred back to the long state. It is further
advantageous when all of the first threshold values U.sub.UL,1 . .
. 5 are selected to be lower than the maximum permissible input
voltage of a connected converter, in particular an inverter. It is
likewise advantageous when all of the second threshold values
U.sub.LL,1 . . . 5 are selected to be greater than the MPP voltage
at the specified conditions so that is it ensured that all of the
strings are in a long state when the photovoltaic field is in the
MPP. A loss of power is hereby prevented by the method according to
the disclosure and by the circuit according to the disclosure for
voltage limitation.
* * * * *